The function of the nervous system is defined by specific patterns of connections between neurons. The molecular determinants of synaptic specificity are largely unknown, however. In the nematode, C. elegans, UNC-4 (homeodomain transcription factor) and its co-repressor, UNC- 37/Groucho, function to ensure that VA motor neurons synapse with the appropriate interneuron partners, unc-4 and unc-37 mutants display a movement defect due to the miswiring of VA motor neurons with inputs from interneurons normally reserved for their lineal sister cells, the VB motor neurons. We propose that UNC-4/UNC-37 preserve normal inputs to VA motor neurons by repressing B motor neuron-specific genes that induce B-type synaptic inputs. The goal of this project is to identify these UNC-4 target genes and to establish their mode of action. FACS will be used to isolate motor neurons expressing A-type or B-type GFP markers from wildtype, unc-4 and unc-37 backgrounds. Labeled mRNA from these cells will be used to interrogate the Affymetrix C. elegans microarray. Authentic unc-4 regulated genes should be normally expressed in Btype motor neurons and upregulated in A-type motor neurons in unc-4 and unc-37 mutants. Transgenic animals expressing GFP reporter genes will be constructed to assess in vivo expression and unc-4 and unc-37 regulation. Presumptive unc-4 target genes will be genetically ablated and potential changes in synaptic connectivity examined by high resolution microscopy. The conservation of UNC-4 and UNC-37 expression in the vertebrate spinal cord suggests that the target genes this study reveals may also regulate synaptic choice in more complex nervous systems.